1. Effect on Hot-Rolled SPA-H
Baseline yield strength: Hot-rolled SPA-H has a minimum yield strength of 345 MPa (per JIS G 3125), with typical actual values ranging from 360–420 MPa.
Post-normalizing change: Yield strength decreases by 5–10%, dropping to a typical range of 340–380 MPa.
Mechanism: Normalizing involves heating to 880–920°C (above austenitization) and air-cooling, which transforms coarse, uneven grains from hot rolling into fine, equiaxed ferrite-pearlite grains. Fine grains improve toughness but reduce yield strength slightly compared to the coarser, stress-induced grain structure of as-rolled steel.
2. Effect on Cold-Rolled SPA-H
Baseline yield strength: Cold-rolled SPA-H has a yield strength of 415–450 MPa (20–30% higher than hot-rolled, due to work hardening).
Post-normalizing change: Yield strength decreases by 12–20%, falling to a typical range of 360–380 MPa.
Mechanism: Normalizing reverses part of the work hardening from cold rolling by recrystallizing the distorted, elongated grain structure. This eliminates internal stresses that contribute to high yield strength, restoring the steel to a more balanced state between strength and ductility.
3. Key Notes
The yield strength reduction is predictable and controlled-normalized SPA-H still meets or exceeds the JIS minimum requirement of 345 MPa, so it retains its classification as a high-strength weathering steel.
The reduction is intentional: it is the cost of improving toughness (30–50% increase in impact energy) and ductility (elongation rises by 8–12% for hot-rolled, 3–5% for cold-rolled), making the steel more weldable and formable.
